Chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs) have many uses, one of which is as a refrigerant. In recent years it has been pointed out that certain CFC and HCFC refrigerants released into the atmosphere may adversely affect the stratospheric ozone layer. Accordingly, there is a demand for the development of refrigerants that have a lower ozone depletion potential than existing refrigerants while still achieving an acceptable performance in refrigeration applications. Hydrofluorocarbons (HFCs) have been suggested as replacements for CFCs and HCFCs because HFCs, which have no chlorine, have been found to have no ozone depletion potential.
The air conditioning industry has looked to find environmentally acceptable refrigerants to replace CFCs and HCFCs in refrigeration applications. Ideally, replacement refrigerant compositions should have the same thermodynamic properties as the composition being replaced, as well as chemical stability, low toxicity, non-flammability and efficiency-in-use. Unfortunately, single component replacement refrigerants are often unable to provide all of the desired properties. In order to match the properties of the refrigerants being replaced, blends of environmentally acceptable refrigerants have been developed to achieve the best possible performance, capacity, efficiency and safety, as well as minimal cost.
When a liquid is heated above its boiling point, it becomes a vapor, and when a vapor is cooled below its condensation point, it becomes a liquid. For pure, single component fluids the boiling point and condensation point temperatures at a given pressure are the same, and the composition of such a fluid is the same in its vapor and liquid states. Fluids can also change state due to a change of pressure. When the pressure on a liquid is lowered below the vaporization pressure it becomes a vapor, and when the pressure is increased above its condensation pressure, it becomes a liquid. For a pure, single component fluid the vaporization and condensation point pressures at a given temperature are the same, and the composition of such a fluid remains constant.
However, for blends of fluids having different thermodynamic properties, such as refrigerant blends, the relationship between vaporization and condensation is more complex. In such fluid mixtures, boiling or condensation may occur over a range of temperatures rather than at a single fixed point. For example, for non-azeotropic blends (also referred to as zeotropic blends) as the temperature of such a fluid mixture in liquid state is raised, the lower boiling-point components boil off preferentially. The point at which the liquid first begins to vaporize is referred to as the bubble point, i.e. the point at which bubbles first form. The bubble point can be expressed as the temperature above which a constant pressure liquid begins to vaporize, or it can be expressed as the pressure below which a constant temperature liquid begins to vaporize, also referred to as the bubble point pressure. Conversely, for such a blend in vapor state, as the temperature of the vapor is lowered, the highest condensation temperature components begin to condense first. The point at which vapor first begins to condense is referred to as the dew point. The dew point can be expressed as the temperature below which a constant pressure vapor begins to vaporize, or it can be expressed as the pressure above which a constant temperature vapor begins to condense, also referred to as the dew point pressure. Thus, a fluid blend begins to vaporize at its bubble point, and completes the vaporization at its dew point, and vice versa.
Because of the different boiling points of the components of such blends, the fluids tend to segregate or fractionate during boiling. That is, as the temperature increases, the lower boiling point components vaporize preferentially. This results in the vapor having a higher concentration of the lower boiling components than the liquid, and a lower concentration of the higher boiling components. This effect is referred to as segregation or fractionation. As a result, when such a fluid blend is stored in a closed container in which there is a vapor space above a quantity of liquid, the composition of the vapor is different from that of the liquid. When such blends are withdrawn or leak from the container in which they are stored, fractionation can take place, with accompanying changes in composition. This effect is demonstrated in the Example set forth below. Composition changes of the mixture can be quite significant, and even relatively small composition changes cannot be tolerated in certain circumstances. Such changes can cause a refrigerant to have a composition outside of specified limits, to have different performance properties or even to become hazardous, such as by becoming flammable.
The range between the bubble and dew points is often referred to as the "glide".
A refrigerant blend may be considered to have a "high glide" if the range from the bubble point to the dew point is greater than about 1.degree. F. (about 0.5.degree. C.) at constant pressure. The problem of fractionation is a particular problem for high-glide refrigerants because of the greater tendency of the low and high boiling point components to segregate. On the other hand, pure single component fluids have zero glide. The composition of the initial vapor is the same as that of the final vapor as the liquid boils off. Therefore, they do not experience the compositional changes of high-glide fluid blends during vaporization.
In the refrigeration industry, refrigerant blends often need to be added to equipment in the field. However, it has been particularly difficult to dispense such blends from existing containers due to the problem of fractionation. There is a recognized need for a portable container or package capable of storing and dispensing such fluid blends, particularly high-glide refrigerant blends, while maintaining the blend's components in substantially uniform proportions.
ASHRAE (American Society of Heating, Refrigeration and Air Conditioning Engineers) has recognized these problems and has tried to examine the effect of the shift in composition and to evaluate blends accordingly. For example, a detailed discussion of 20 such blends may be found in Didion, et al., "The Role of Refrigerant Mixtures as Alternatives", pages 57-69, Proceedings of ASHRAE's 1989 CFC Technology Conference, Sep. 27-28,1989, incorporated herein by reference. A difficulty that has recently gained recognition is that even during normal use, when the refrigerant is dispensed as a liquid, fractionation can change the refrigerant blend composition 25 sufficiently that the blend will no longer be within the tolerance set in the evaluation.
This problem has been observed for refrigerant blends such as R-407C and R-409A. To a lesser extent even R-410A and R-507 also show some composition shifts during use. The problem is even more pronounced when the refrigerant leaks from a container in vapor form. As previously indicated, the effects of fractionation are demonstrated by the Example set forth below.
The composition as well as data and safety classifications of refrigerant blends are set forth in Table 2 of ANSI/ASHRAE Standard 34-1997, incorporated herein by reference. The four above-identified blends are listed as having the following nominal compositions (weight percentages):
R-407C R-32/125/134a 23/25/52 R-409A R-22/124/142b 60/25/15 R-410A R-32/125 50/50 R-507 R-125/143a 50/50
It may be noted that R-507 is identified in the Standard as being azeotropic, which would mean that the fluid has a low glide, and that the liquid and vapor have the same composition when in equilibrium. However, it has been observed that at some conditions of temperature and pressure R-507 shows fractionation. Therefore, this blend, as well as 10 others listed as being azeotropic, may be subject to fractionation at certain conditions of temperature and pressure.
One way to prevent fractionation is to have only one phase present in a cylinder containing a refrigerant blend. This presents several problems. If a rigid cylinder were filled with only liquid, an increase in temperature could cause it to rupture due to static 15 pressure. Conversely, if the temperature of the liquid were decreased, a vapor space would have to form above the liquid, or the cylinder would have to contract. If only vapor were used, the volume of the cylinder would have to be so large it would not be practical.
Another practice to prevent fractionation is to employ single-use packaging. That is, the cylinder contains the exact quantity of material needed for one application and the contents are exhausted in that single use. This is not practical in the air conditioning and refrigeration industry due to the wide variety of equipment and charge sizes required. The number of differently sized packages that would be necessary would be too large to stock and manage economically.
Yet another method is to remove only liquid from the cylinder. This causes far less fractionation than removing vapor but the composition can still shift to an unacceptable degree, as demonstrated by the Example set forth below. One improvement on this method entails mixing some vapor with the liquid as it is removed using a perforated dip tube as described in U.S. Patent No. 3,656,657. This method is not used widely, probably due to its dependency on flow rate.
Accordingly, it is an object of the invention to provide a method and apparatus for storing and dispensing a blend of fluids having different thermodynamic properties without the composition changing.
Other objects and advantages of the invention will become apparent from the following description of the invention.